Luster or effect pigments are used in many areas, for example in automotive coatings, decorative coatings, plastics pigmentation, paints, printing inks, and cosmetics.
The optical effect is based on the directed reflection of light at predominantly sheet-like, parallel-oriented, metallic or strongly refractive pigment particles. Depending on the composition of the pigment platelets, there are interference, reflection and absorption phenomena which create angular-dependent color and lightness effects.
Metallic effect pigments are all of the platelet-shaped substrates known to the skilled person, examples being aluminium platelets/flakes or metal oxide-coated aluminium platelets/flakes.
Platelet-shaped aluminium pigments having a coating of iron oxide are well known and described e.g. in EP 0 033 457. They belong to the class of effect pigments which, by virtue of their particular color properties, have found wide use in the coloration of coatings, paints, printing inks, plastics, ceramic compositions and glazes and decorative cosmetic preparations.
Iron oxide coated aluminium pigments derive their particular optical profile from a combination of specular reflection at the surface of the aluminium platelet, selective light absorption in the iron oxide layer and light interference at the film-like surfaces of the iron oxide layer. Light interference leads to a color which is mainly determined by the thickness of the iron oxide coating layer. Dry pigment powders therefore exhibit the following hues in air with increasing iron oxide layer thickness which are classified as due to 1st order or 2nd order interference:
1st order interference colors: pale yellow, green-gold, gold, reddish-gold, red, violet, grayish-violet;
2nd order interference colors: yellow, gold, reddish-gold, red-gold, red.
Iron oxide coated aluminium pigments are very bright and opaque, which is why they are widely used in automotive coatings. The pigments customarily used in this field are based on aluminium platelets and exhibit a metallic mirror effect.
Metal oxide layers of effect pigments can be provided on the metallic substrate particles by gas phase decomposition of volatile metal compounds in the presence of oxygen and/or water vapor or by a wet-chemical coating process (e.g. sol-gel process).
EP 0 033 457 A2 describes a process for the preparation of colored effect pigments comprising a metallic substrate whose surface is at least partially covered with an iron oxide, wherein iron pentacarbonyl is oxidized to iron oxide in a fluidized bed of the metallic substrates with oxygen at above 100° C.
In wet-chemical preparation methods, metal oxide containing layers can be applied by hydrolytic reaction of appropriate metal salts, e.g. iron(III) salts such as iron(III) chloride and sulfate, or hydrolysable organometallic compounds.
Details about the preparation of a metal oxide coating layer on a metal-based substrate of an effect pigment are provided e.g. in EP 0 708 154 A2.
Typically, a metal oxide layer prepared via a wet-chemical preparation method may contain hydroxyl groups due to incomplete condensation reaction of hydrolysed precursor species or bound water. For coloristic reasons, conversion of the hydroxide-containing oxide layer into the fully condensed oxide layer and/or removal of bound water is preferred so as to avoid any undesired pigment color shift in the applied pigment-containing product. This is typically accomplished by drying in a hot gas stream.
However, if the metal substrate of the effect pigment comprises aluminium, such a drying step may trigger an aluminothermic reaction.
Aluminothermic reactions are highly exothermic chemical reactions between aluminium acting as a reducing agent and a metal oxide such as iron oxide or titanium oxide. The most prominent example is the thermite reaction between aluminium and iron oxide. However, aluminium may also react with a titanium oxide or other oxides such as SiO2.